AXT Systems

APM-3 Signal Metering Module
APM-3 Level Metering Module
APM-3 Level Metering Module
The APM-3 metering module was originally designed as a simple power level meter for BTL power amplifiers (such as the PA-3 and PA-4). However with simple circuit adjustments is was also used as a line level peak meter and even a gain-reduction level meter for a limiter design.

It is powered with +/-12V rails, which can be up to +/-16V with adjustments to the comparator resistor values, since the chosen values use the +12V rail as the reference. It uses similar circuitry as the MPM-3C metering module, with only 4 LEDs for compactness.

A trimmer pot is used to calibrate the meter for the chosen input signal level. Calibration is usually performed at the lowest level (as shown on the circuit diagram)

The LED colours can be chosen to suit the application.
The module is mostly SMD to keep the size small. The pitches are 1.25mm or greater so should not present a challenge to constructors.

There are several variants described here for different applications.Additional variants can be created for alternative applications.

V01: BTL audio power meter, 48V peak output.
V02: BTL audio power meter, 40V peak output.
V04: Line level peak meter.

Not that the V04 circuit for line level peak metering has a different input signal polarity than marked on the PCB.